I take a Helly Hansen packable shell. Zips into its own pocket and is 7x7x3". If I'm stowing it for the duration of the heat of the day, I toe-strap it to seat rails. If I had aero bars, I'd consider strapping it to them somehow.
Short-term storage techniques include clipping it to my stem with a carabiner or just stuffing it in a bottle cage. I do these sometimes when I'm in the flatlands and can see that I'm gonna hit another storm in a half hour.
Last edited by LesterOfPuppets; 03-04-10 at 12:15 PM.